The present invention relates to an MR imaging method and an MRI (Magnetic Resonance Imaging) apparatus, and more particularly to an MR imaging method and an MRI apparatus capable of improving picture quality without increasing the number of times of data collection and restraining artifacts resulting from differences in noise structure between parts differing in the number of times of addition.
In an MR imaging method, it is usual to configure k spaces out of views from the first view through a Vth view which differ in the quantity of phase encoding, repeatedly collect data by Xn times of addition in an nth (n=1xcx9cV) view, and use data An resulting from arithmetic averaging as the data of the nth view for image reconfiguration.
According to the related art, the number of times of addition Xn may be constant all the time as illustrated in FIG. 1, varied linearly to become greater as the absolute value of the quantity of phase encoding decreases as illustrated in FIG. 2, or varied stepwise to become greater as the absolute value of the quantity of phase encoding decreases as illustrated in FIG. 3.
By an MR imaging method according to the related art, whereby the number of times of addition Xn is to be kept constant all the time as shown in FIG. 1, there is a problem that picture quality is little improved relative to an increase in the overall number of times of data collection because parts where the absolute value of the quantity of phase encoding is large, which little contribute to picture quality, and parts where the absolute value of the quantity of phase encoding is small, which greatly contribute to picture quality, are added the same number of times.
On the other hand, where the number of times of addition is varied to become greater as the absolute value of the quantity of phase encoding decreases as shown in FIG. 2 or FIG. 3, picture quality can be improved without increasing the overall number of times of data collection because parts where the absolute value of the quantity of phase encoding is large, which little contribute to picture quality, are added a fewer number of times and because parts where the absolute value of the quantity of phase encoding is small, which greatly contribute to picture quality, are added a greater number of times.
However, where the variation is linear as shown in FIG. 2, there is a problem that the advantage of improving picture quality without increasing the overall number of times of data collection is not sufficient because, although the number of times of addition is maximized in the view where the absolute value of the quantity of phase encoding is xe2x80x9c0xe2x80x9d, the number of times of addition linearly decreases in a part away from that view even by a minimal distance.
On the other hand, where the variation is stepwise as shown in FIG. 3, the advantage of improving picture quality without increasing the overall number of times of data collection is increased because the number of times of addition is maximized not only in the view where the absolute value of the quantity of phase encoding is xe2x80x9c0xe2x80x9d but also in neighboring views.
However, since the variation in the number of times of addition is not continuous, there is a problem that differences in noise structure between parts differing in the number of times of addition are apt to invite the emergence of artifacts.
Therefore, an object of the present invention is to provide an MR imaging method and an MRI apparatus capable of improving picture quality without increasing the overall number of times of data collection and restraining artifacts resulting from differences in noise structure between parts differing in the number of times of addition.
In its first aspect, the invention provides an MR imaging method whereby k spaces are configured out of views from the first view, where the quantity of phase encoding takes on the largest negative (or positive) value, through a Vth view, where the quantity of phase encoding takes on the largest positive (or negative) value, and data An resulting from arithmetic averaging as the data of the nth view resulting from repeated collection of data by Xn times of addition in an nth (nxe2x88x921xcx9cV) view are used for image reconfiguration, characterized in that the number of times of addition Xn is varied to become greater as the absolute value of the quantity of phase encoding decreases and to cause all or part of it to follow either a Hamming function or a Hanning function.
The above-described MR imaging method according to the first aspect can improve picture quality without increasing the overall number of times of data collection because the number of times of addition Xn is varied to become greater as the absolute value of the quantity of phase encoding decreases and to cause all or part of it to follow either a Hamming function or a Hanning function, and accordingly the number of times of addition is increased not only in the view where the absolute value of the quantity of phase encoding is xe2x80x9c0xe2x80x9d but also in neighboring views. Moreover, since the variation in the number of times of addition is continuous, artifacts resulting from differences in noise structure between parts differing in the number of times of addition can be restrained.
In its second aspect, the invention provides an MR imaging method whereby k spaces are configured out of views from the first view, where the quantity of phase encoding takes on the largest negative (or positive) value, through a Vth view, where the quantity of phase encoding takes on the largest positive (or negative) value, and data An resulting from arithmetic averaging as the data of the nth view resulting from repeated collection of data by Xn times of addition in an nth (n=1xcx9cV) view are used for image reconfiguration, characterized in that, where the reference number of times of addition is N:
(1) the number of times of addition from the first view till the (V/8)-th view is (Nxe2x88x92N/2);
(2) the number of times of addition from the (Vxe2x88x92V/8+1)-th view till the Vth view is (Nxe2x88x92N/2);
(3) the number of times of addition from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th view is (N+N/2); and
(4) the number of times of addition from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th is a value resulting from the subtraction of (Nxe2x88x92N/2) from the earlier calculated number of times of addition, multiplication of the balance by either a Hamming function or a Hanning function, addition of (Nxe2x88x92N/2) to the product, and discrete processing of the sum.
By the above-described MR imaging method according to the second aspect, the total number of views V is equally divided into eight, and the areas of the two ends of each V/8, where the absolute value of the quantity of phase encoding is the greatest, are reduced in the number of times of addition by xc2xd of the reference number of times N, because their contributions to picture quality is small. On the other hand, the remaining middle 6V/8 areas are increased in the number of times of addition by xc2xd of the reference number of times N, because their contributions to picture quality are great, and further to alleviate the non-continuity of the variations of the number of times of addition, the number of times of addition is varied so as to follow either a Hamming function or a Hanning function. This makes possible improvement of picture quality without increasing the overall number of times of data collection because the number of times of addition is increased not only in the view where the absolute value of the quantity of phase encoding is xe2x80x9c0xe2x80x9d but also in neighboring views. Furthermore, as the variation in the number of times of addition is continuous in the part from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th view, whose contributions to picture quality are great, artifacts resulting from differences in noise structure between parts differing in the number of times of addition can be restrained.
In its third aspect, the invention provides an MR imaging method whereby k spaces are configured out of views from the first view, where the quantity of phase encoding takes on the largest negative (or positive) value, through a Vth view, where the quantity of phase encoding takes on the largest positive (or negative) value, and data An resulting from arithmetic averaging as the data of the nth view resulting from repeated collection of data by Xn times of addition in an nth (n=1xcx9cV) view are used for image reconfiguration, characterized in that, where the reference number of times of addition is N:
(1) the number of times of addition from the first view till the (V/8)-th view is (Nxe2x88x92N/2);
(2) the number of times of addition from the (Vxe2x88x92V/8+1)-th view till the Vth view is (Nxe2x88x92N/2);
(3) the number of times of addition from the (V/2xe2x88x92V/8+1)-th view till the (V2+V/8)-th view is (N+N/2);
(4) the number of times of addition from the (V/8+1)-th view till the (V/2xe2x88x92V/8)-th view is a number of times represented by a straight line linking the (V/8)-th view with the (V/2xe2x88x92V/8+1)-th;
(5) the number of times of addition from the (V/2+V/8+1)-th view till the (Vxe2x88x92V/8)-th view is a number of times represented by a straight line linking the (V/2+V/8)-th view with the (Vxe2x88x92V/8+1)-th; and
(6) the number of times of addition from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th is a value resulting from the subtraction of (Nxe2x88x92N/2) from the earlier calculated number of times of addition, multiplication of the balance by either a Hamming function or a Hanning function, addition of (Nxe2x88x92N/2) to the product, and discrete processing of the sum.
By the above-described MR imaging method according to the third aspect, the total-number of views V is equally divided into eight, and the areas of the two ends of each V/8, where the absolute value of the quantity of phase encoding is the greatest, are reduced in the number of times of addition by xc2xd of the reference number of times N, because their contributions to picture quality is small. On the other hand, the remaining middle 6V/8 areas are increased in the number of times of addition by xc2xd of the reference number of times N, because their contributions to picture quality are great. Then, the areas in which the number of times of addition has been reduced by xc2xd of the reference number of times N and the areas in which the number of times of addition has been increased by xc2xd of the reference number of times N are linked by a straight line to equalize the overall number of times of addition. Further to alleviate the non-continuity of the variations of the number of times of addition, the number of times of addition in the middle 6V/8 areas is varied so as to follow either a Hamming function or a Hanning function. This makes possible improvement of picture quality without increasing the overall number of times of data collection because the number of times of addition is increased not only in the view where the absolute value of the quantity of phase encoding is xe2x80x9c0xe2x80x9d but also in neighboring views. Furthermore, as the variation in the number of times of addition is continuous in the part from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th view, whose contributions to picture quality are great, artifacts resulting from differences in noise structure between parts differing in the number of times of addition can be restrained.
In its fourth aspect, the invention provides an MRI apparatus comprising an RF pulse transmitting means, a slope pulse applying means and an NMR signal receiving means, wherein those means are controlled so as to use for image reconfiguration data An resulting from repeated collection by Xn times of addition and the arithmetic averaging of data in an nth view (n=1xcx9cV) of k spaces configured out of views from the first view, where the quantity of phase encoding takes on the largest negative (or positive) value, through a Vth view, where the quantity of phase encoding takes on the largest positive (or negative) value, characterized in that it is further provided with a determination means for the number of times of addition to vary the number of times of addition Xn to become greater as the absolute value of the quantity of phase encoding decreases and to cause all or part of it to follow either a Hamming function or a Hanning function.
The above-described MRI apparatus according to the fourth aspect of the invention can appropriately implement the MR imaging method according to the first aspect described above.
In its fifth aspect, the invention provides an MRI apparatus comprising an RF pulse transmitting means, a slope pulse applying means and an NMR signal receiving means, wherein those means are controlled so as to use for image reconfiguration data An resulting from repeated collection by Xn times of addition and the arithmetic averaging of data in an nth view (n=1xcx9cV) of k spaces configured out of views from the first view, where the quantity of phase encoding takes on the largest negative (or positive) value, through a Vth view, where the quantity of phase encoding takes on the largest positive (or negative) value, characterized in that it is further provided with a determination means for the number of times of addition to perform, where the reference number of times of addition is N, addition (Nxe2x88x92N/2) times from the first view till the (V/8)-th view; addition (Nxe2x88x92N/2) times from the (Vxe2x88x92V/8+1)-th view till the Vth view; addition (N+N/2), times from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th view; and addition, from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th, as many times as a value resulting from the subtraction of (Nxe2x88x92N/2) from the earlier calculated number of times of addition, multiplication of the balance by either a Hamming function or a Hanning function, addition of (Nxe2x88x92N/2) to the product, and discrete processing of the sum.
The above-described MRI apparatus according to the fifth aspect of the invention can appropriately implement the MR imaging method according to the second aspect described above.
In its sixth aspect, the invention provides an MRI apparatus comprising an RF pulse transmitting means, a slope pulse applying means and an NMR signal receiving means, wherein those means are controlled so as to use for image reconfiguration data An resulting from repeated collection by Xn times of addition and the arithmetic averaging of data in an nth view (n=1xcx9cV) of k spaces configured out of views from the first view, where the quantity of phase encoding takes on the largest negative (or positive) value, through a Vth view, where the quantity of phase encoding takes on the largest positive (or negative) value, characterized in that it is further provided with a determination means for the number of times of addition to perform, where the reference number of times of addition is N, addition (Nxe2x88x92N/2) times from the first view till the (V/8)-th view; addition (Nxe2x88x92N/2) times from the (Vxe2x88x92V/8+1)-th view till the Vth view; addition (N+N/2) times from the (V/2xe2x88x92V/8+1)-th view till the (V/2+V/8)-th view; addition, from the (V/8+1)-th view till the (V/2xe2x88x92V/8)-th view, a number of times represented by a straight line linking the (V/8)-th view with the (V/2xe2x88x92V/8+1)-th; addition, from the (V/2+V/8+1)-th view till the (Vxe2x88x92V/8)-th view, a number of times represented by a straight line linking the (V/2+V/8)-th view with the (Vxe2x88x92V/8+1)-th; and addition, from the (V/8+1)-th view till the (Vxe2x88x92V/8)-th, as many times as a value resulting from the subtraction of (Nxe2x88x92N/2) from the earlier calculated number of times of addition, multiplication of the balance by either a Hamming function or a Hanning function, addition of (Nxe2x88x92N/2) to the product, and discrete processing of the sum.
The above-described MRI apparatus according to the sixth aspect of the invention can appropriately implement the MR imaging method according to the third aspect described above.
By the MR imaging method and the MRI apparatus according to the present invention, picture quality can be efficiently improved without increasing the overall number of times of data collection. Furthermore, artifacts resulting from differences in noise structure between parts differing in the number of times of addition Xn can be restrained.